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If you are able to sustain it siroria hits harder and that fight is literally about pushing phases as fast as possible with single target DPS and since you don't have to move siroria = more damage.
If there is one fight in VSS HM to go full glass cannon, it's the fire boss. This is 100% a judgement call. Siroria is more DPS on this fight, assuming you can keep your stacks up and sustain is not an issue. The only exception might be is if you are tasked as a range DPS to go out and kill adds at some point. If so, you would probably do better with FG.
If you have plenty of time to swap gear, definitely swap to Siroria for stationary fights (particularly fire boss vss hm). Blood for Blood makes magicka sustain easy so there’s not as much need for False God as there used to be.
If you’re on console and doing a score run, it’ll be up to your raid group because swapping too much gear/skills really hurts score runs. I’ve seen groups take 30sec+ on gear swaps to fight a boss before because they are swapping too many things.
Generally on console you should run your boss-killing setup for the whole trial. For add pulls only change 2-3 skills on your Frontbar and swap the Frontbar staff into a Master Lightning Staff for shock clench spam. It’s highly effective and takes only 10-15sec to swap out at a boss.